A penetrometer is an apparatus used for exploring the ground driving a probe placed at the lower end of a set or string of tubes into the ground. The measurements which may be carried out by means of the penetrometer are in particular the displacement of the probe, i.e. the penetration depth of the tubes, the resistance exerted on the point of the probe, on its lateral surface, and on the entire set of tubes.
These measurements are normally undertaken during static operation of the apparatus, which consists of exerting considerable pressure on the top of the set of tubes, by a powerful hydraulic ram. When it is necessary to drive the tubes into very hard ground, penetration of the probe is achieved by means of a pile driver. This operation is termed dynamic and when it is being carried out it is necessary to stop making measurements of resistance.
In case of both static and dynamic operation it is important to know exactly the penetration of the tubes at any instant. During static operation measurement of the various forces exerted on the probe and on the entire set of tubes is significant only if it is related to the depth to which the probe is driven. During dynamic operation, it is customary to count the number of hammer blows necessary to obtain a given penetration.
At present the means used for measuring the penetration of the tube is rudimentary: the tubes are counted and the length of tubes driven in is measured directly by marking with chalk, for example every 10 centimeters. A slightly more sophisticated system described in French Pat. No. 1,077,660 of 27 Mar. 1953 to Marius ANDINA and Gaston BAUER, provides a fixed graduated rule arranged vertically, along which moves a pointer supported by the head containing the measuring apparatus, which itself is disposed at the top of the set of tubes. This device makes it possible to follow the penetration of the probe into the ground, however it has two main drawbacks. First of all it does not indicate the total penetration from the beginning of the test, but only shows partial penetrations which must be added, thus involving the risk of error. Secondly it does not facilitate recording of the penetration.
At the present time no automatic apparatus exists for measuring and recording the penetration of the tubes without manual intervention and the risk of error. An apparatus of this type would be very advantageous since, with currently available electronic means for processing the information, it would make it possible to record and possibly simultaneously to print out the value of the forces during static operation or the number of hammer blows during dynamic operation and the corresponding depth of the probe.
To provide an apparatus of this type, it is not sufficient directly to measure the descent of the anvil. Indeed, it may happen that it is necessary to raise the anvil during the tests, without adding tubes, for example subsequent to an incorrect movement or for handling requirements. The anvil must then be relowered without this corresponding to driving-in of the tubes, so that simple measurement of the total descent of the anvil no longer corresponds to the penetration of the tubes.
Measurement of the descent of the head containing the measuring apparatus would possibly eliminate this drawback during static operation. However, this solution must be disregarded since during dynamic operation, the aforesaid head is withdrawn.
It thus appears necessary to measure and record the descent of the anvil and not of the head containing the measuring apparatus, by a device which adds solely the useful movements of the anvil, i.e. those during which the anvil drives the set of tubes and which disregards parasitic movements resulting from incorrect operation or movements for releasing the anvil carried out solely for handling reasons.